This invention relates to novel amino acid analogs having specific and selective binding in a biological system, particularly brain and systemic tumors, and capable of being used for positron emission tomography (PET) and single photon emission (SPECT) imaging methods.
The development of radiolabeled amino acids for use as metabolic tracers to image tumors using positron emission tomography (PET) and single photon emission computed tomography (SPECT) has been underway for some time. Although radiolabeled amino acids have been applied to a variety of tumor types, their application to intracranial tumors has received considerable attention due to potential advantages over other imaging modalities. After surgical resection and/or radiotherapy of brain tumors, conventional imaging methods such as CT and MRI do not reliably distinguish residual or recurring tumor from tissue injury due to the intervention and are not optimal for monitoring the effectiveness of treatment or detecting tumor recurrence [Buonocore, E (1992), Clinical Positron Emission Tomography. Mosby-Year Book, Inc. St. Louis, Mo., pp 17-22; Langleben, D D et al. (2000), J. Nucl. Med. 41:1861-1867].
The leading PET agent for diagnosis and imaging of neoplasms, 2-[18F]fluorodeoxyglucose (FDG), has limitations in the imaging of brain tumors. Normal brain cortical tissue shows high [18F]FDG uptake as does inflammatory tissue which can occur after radiation or surgical therapy; these factors can complicate the interpretation of images acquired with [18F]FDG [Griffeth, L K et al. (1993), Radiology. 186:37-44; Conti, P S (1995)].
A number of reports indicate that PET and SPECT imaging with radiolabeled amino acids better define tumor boundaries within normal brain than CT or MRI allowing better planning of treatment [Ogawa, T et al. (1993), Radiology. 186: 45-53; Jager, P L et al. (2001), Nucl. Med., 42:432-445]. Additionally, some studies suggest that the degree of amino acid uptake correlates with tumor grade, which could provide important prognostic information [Jager, P L et al. (2001) J. Nucl. Med. 42:432-445].
Amino acids are required nutrients for proliferating tumor cells. A variety of amino acids containing the positron emitting isotopes carbon-11 and fluorine-18 have been prepared. They have been evaluated for potential use in clinical oncology for tumor imaging in patients with brain and systemic tumors and may have superior characteristics relative to 2-[18F]FDG in certain cancers. These amino acid candidates can be subdivided into two major categories. The first category is represented by radiolabeled naturally occurring amino acids such as [11C]valine, L-[11C]leucine, L-[11C]methionine (MET) and L[1-11C]tyrosine, and structurally similar analogues such as 2-[18F]fluoro-L-tyrosine and 4-[18F]fluoro-L-phenylalanine. The movement of these amino acids across tumor cell membranes predominantly occurs by carrier mediated transport by the sodium-independent leucine type “L” amino acid transport system. The increased uptake and prolonged retention of these naturally occurring radiolabeled amino acids into tumors in comparison to normal tissue is due in part to significant and rapid regional incorporation into proteins. Of these radiolabeled amino acids, [11C]MET as been most extensively used clinically to detect tumors. Although [11C]MET has been found useful in detecting brain and systemic tumors, it is susceptible to in vivo metabolism through multiple pathways, giving rise to numerous radiolabeled metabolites. Thus, graphical analysis with the necessary accuracy for reliable measurement of tumor metabolic activity is not possible. Studies of kinetic analysis of tumor uptake of [11C]MET in humans strongly suggest that amino acid transport may provide a more sensitive measurement of tumor cell proliferation than protein synthesis.
The shortcomings associated with [11C]MET may be overcome with a second category of amino acids. These are non-natural amino acids such as 1-aminocyclobutane-1-[11C]carboxylic acid ([11C]ACBC). The advantage of [11C]ACBC in comparison to [11C]MET is that it is not metabolized. A significant limitation in the application of carbon-11 amino acids for clinical use is the short 20-minute half-life of carbon-11. The 20-minute half-life requires an on-site particle accelerator for production of the carbon-11 amino acid. In addition only a single or relatively few doses can be generated from each batch production of the carbon-11 amino acid. Therefore carbon-11 amino acids are poor candidates for regional distribution for widespread clinical use.
In order to overcome the physical half-life limitation of carbon-11, we have recently focused on the development of several new fluorine-18 labeled non-natural amino acids, some of which have been disclosed in U.S. Pat. Nos. 5,808,146 and 5,817,776, both of which are incorporated herein by reference. These include anti-1-amino-3-[18F]fluorocyclobutyl-1-carboxylic acid (anti-[18F]FACBC), syn-1-amino-3-[18F]fluorocyclobutyl-1-carboxylic acid (syn-[18F]FACBC), syn- and anti-1-amino-3-[18F]fluoromethyl-cyclobutane-1-carboxylic acid (syn- and anti-[18F]FMACBC). These fluorine-18 amino acids can be used to image brain and systemic tumors in vivo based upon amino acid transport with the imaging technique Positron Emission Tomography (PET). Our development involved fluorine-18 labeled cyclobutyl amino acids that move across tumor capillaries by carrier-mediated transport involving primarily the “L” type large, neutral amino acid and to a lesser extent the “A” type amino acid transport systems. Our preliminary evaluation of cyclobutyl amino acids labeled with positron emitters, which are primarily substrates for the “L” transport system, has shown excellent potential in clinical oncology for tumor imaging in patients with brain and systemic tumors. The primary reasons for proposing 18F-labeling of cyclobutyl/branched amino acids instead of 11C (t1/2=20 min.) are the substantial logistical and economic benefits gained with using 18F instead of 11C-labeled radiopharmaceuticals in clinical applications. The advantage of imaging tumors with 18F-labeled radiopharmaceuticals in a busy nuclear medicine department is primarily due to the longer half-life of 18F (t1/2=110 min.). The longer half-life of 18F allows off-site distribution and multiple doses from a single production lot of radio tracer. In addition, these non-metabolized amino acids may also have wider application as imaging agents for certain systemic solid tumors that do not image well with 2-[18F]FDG PET. WO 03/093412, which is incorporated herein by reference, further discloses examples of fluorinated analogs of α-aminoisobutyric acid (AIB) such as 2-amino-3-fluoro-2-methylpropanoic acid (FAMP) and 3-fluoro-2-methyl-2-(methylamino)propanoic acid (N-MeFAMP) suitable for labeling with 18F and use in PET imaging. AIB is a nonmetabolizable α,α-dialkyl amino acid that is actively transported into cells primarily via the A-type amino acid transport system. System A amino acid transport is increased during cell growth and division and has also been shown to be upregulated in tumor cells [Palacín, M et al. (1998), Physiol. Rev. 78: 969-1054; Bussolati, O et al. (1996), FASEB J. 10:920-926]. Studies of experimentally induced tumors in animals and spontaneously occurring tumors in humans have shown increased uptake of radiolabeled AIB in the tumors relative to normal tissue [Conti, PS et al. (1986), Eur. J. Nucl. Med. 12:353-356; Uehara, H et al. (1997), J. Cereb. Blood Flow Metab. 17:1239-1253]. The N-methyl analog of AIB, N-MeAIB, shows even more selectivity for the A-type amino acid transport system than AIB [Shotwell, M A et al. (1983), Biochim. Biophys. Acta. 737:267-84]. N-MeAIB has been radiolabeled with carbon-11 and is metabolically stable in humans [Någren, K et al. (2000), J. Labelled Cpd. Radiopharm. 43:1013-1021].
Although some of the amino acid analogs mentioned above are currently being evaluated as tumor imaging agents in patients with brain and systemic tumors, there is a continued need for a novel imaging agent which can bind tumor cells or tissues with high specificity and selectivity and can readily be prepared in sufficient quantities for tumor imaging with PET and SPECT. As a candidate compound makes the transition from validation studies in cells in vitro and animal models to application in humans, the synthetic methods employed must be adapted to allow routine, reliable production of the compound in large quantities. Towards this end, the present application discloses a series of novel amino acid compositions, methods of synthesizing and using those compounds for tumor imaging with PET and SPECT.